1. Field of the Invention
The invention relates to a working device. In particular, the invention relates to working devices such as a ramming device for compacting soil or a hammer.
2. Description of the Related Art
Known ramming devices of this kind are constructed in such a way that an upper mass which accommodates a motor and a crank mechanism is connected via a set of springs to a working mass essentially forming a working or compacting plate. The rotary motion produced by the motor is converted by the crank mechanism into an oscillating axial motion, which is transmitted via the set of springs to the working plate for the purpose of compacting the soil. The upper mass comprises approximately two thirds and the percussive working mass one third of the total mass of the ramming device, while the respective distances traveled by the upper mass and the working mass are inversely proportional to one another. In this arrangement, the movement of the upper mass is of the order of 25 to 30 mm.
The vibrations of the upper mass are transmitted via a guide hoop to the person guiding the working device and this is very unpleasant, particularly when working for a prolonged period. In this context, vibrations in the horizontal or lateral direction are particularly stressful for the operator. Vibrations in the vertical direction, on the other hand, are necessary for the ramming device to work effectively.
FIG. 2 shows a known ramming device of this kind.
According to the invention, the object is achieved by providing a working device, such as a ramming device for compacting soil or a hammer, that includes a working mass which can be driven backwards and forwards in a linear manner by a motor belonging to an upper mass via a single crank mechanism and a set of springs. The crank mechanism has a crank disk which is driven by the motor and which, eccentrically with respect to its axis of rotation, carries a crank pin. The crank pin is coupled to a connecting rod. The working device also includes a countermass which is carried by the crank disk and which has a center of gravity that is offset from the center of gravity of the crank pin by an angle unequal to 180xc2x0 relative to the axis of rotation of the crank disk.
Arranged on both sides of the piston guide 9 is a set of springs 11 comprising a plurality of springs, the springs each being supported, at their end remote from the piston guide 9, against spring plates 12 secured on the guide tube 10.
The guide tube 10 and the spring plates 12 belong to the working or lower mass of the ramming device. A ramming foot (not shown in FIG. 2), for compacting the soil can be attached to the lower mass. To prevent the ingress of moisture and dirt, the upper mass and the lower mass are connected by a flexible bellows 13.
As can be seen from FIG. 2, the rotary motion of the motor is converted into an oscillating axial motion of the guide piston 7 by the crank mechanism comprising the crank disk 3, the crank pin 4 and the connecting rod 5. This axial motion is transmitted to the guide tube 10 and hence to the lower mass by the set of springs 11 and can be used to compact soil.
The previously known method of damping the vibrations acting on the operator was to decouple the guide hoop from the upper mass mechanically by means of rubber elements. However, the attached drive motor is still subject to high vibrational loading. In this arrangement, it is only possible to improve the vibration damping by highly complex construction.
It is therefore desirable to avoid the occurrence of vibrations of the upper mass from the outset.
DE-A 19 25 870 has disclosed a ramming device in which the drive power of the motor is divided between two crank mechanisms which act simultaneously on a working mass via sets of springs. Countermasses are secured on the crank mechanisms, which rotate in opposite directions, the centrifugal forces of these countermasses balancing one another in the horizontal direction but are added to one another in the vertical direction, thereby reducing the vibration amplitude of the housing of the ramming device. With this device too, however, horizontal vibrations that are stressful for the operator occur.
The object underlying the invention is therefore to specify a working device in which it is possible to reduce horizontal vibrations of the upper mass right at their inception.
According to the invention, the object is achieved by a working device with the features of patent claim 1.
It has surprisingly been found that balancing the crank mechanism, i.e. providing the crank disk carrying the crank pin with a countermass which balances the mass of the crank pin does not lead to the desired improvement in terms of a reduction in the horizontal vibrations of the upper mass. On the contrary, it was found as a complete surprise that it is only possible to reduce the horizontal vibrations effectively if, instead of the center of gravity of the crank pin, the axis of rotation of the crank drive and the center of gravity of the countermass lying on a straight line and the center of gravity thus being offset by 180xc2x0 relative to the axis of rotation of the crank disk, the center of gravity of the countermass is arranged offset from the center of gravity of the crank pin by an angle unequal to 180xc2x0 relative to the axis of rotation of the crank disk.
Although this means that the crank mechanism, taken by itself, generates considerable vibrations, these are superimposed on the vibrations generated by the following components, in particular the guide piston and the spring assemblies, and these vibrations essentially compensate for one another. Consequently, the superimposition of the vibrations leads to considerable stabilization of the upper mass, especially in the horizontal direction.
In a preferred embodiment, the mass and the angular offset of the countermass can be adjusted in such a way, as a function of the characteristics of the set of springs, that the vibrations which are not directed in a working or vertical direction of the working device are minimal. This is possible, for example, during the manufacture or assembly of the crank disk, it being possible to provide an appropriate angular arrangement of the countermass of suitable mass depending on the set of springs used.
In a particularly preferred embodiment, the countermass is formed in one piece with the crank disk. This simplifies assembly and production. An appropriate casting mold can be used when casting the crank disk or an appropriate die when forging the crank disk, this depending on the production method.
In another preferred embodiment, the countermass can be secured on the crank disk. It is particularly advantageous here if the countermass can be secured at various points on the crank disk relative to the crank pin in terms of its angle around the axis of rotation. This has the advantage that fine adjustments can be performed during assembly, leading to a further reduction in the horizontal vibrations of the upper mass. It is likewise possible, without changing the molds or dies used in production, to match the crank disk and the countermass individually to one another, depending on the various sets of springs fitted.
It has proved expedient if the difference between the offset angle and 180xc2x0 is at least 10xc2x0.
In a particularly advantageous embodiment of the invention, the angle between the center of gravity of the countermass and the center of gravity of the crank pin is about 90xc2x0 relative to the axis of rotation of the crank disk.
It has likewise proven expedient if the mass of the countermass is essentially equal to the mass of the crank pin.
In another embodiment of the invention, the mass of the countermass is greater, indeed preferably considerably greater, than that of the crank pin.
This makes it possible to use the countermass to balance the moving masses of the connecting rod, the guide piston and the piston guide as well.